Conventionally, diesel power generators are used for providing power to island areas that are isolated from large power systems on mainland areas. When power is provided by using diesel power generators, there is no technical difficulty since output power is autonomously controlled according to changes in loads.
However, recently, renewable energy sources including sunlight, wind power, etc. have been introduced in this area for supplying environmentally friendly electric power, and a battery is necessarily introduced together to compensate for the unstable output of such renewable energy. Herein, a “stand-alone micro-grid” may be defined as a system independently providing power from power generation sources such as battery, diesel power generator, etc. which are separated from a large scale power system.
Herein, due to the unstable output of renewable energy, the stand-alone micro-grid needs to maintain a frequency by adjusting in real time power supply according to demand. Generally, since the outputs of photovoltaic power generation, wind power generation, and other renewable energy source fluctuate, a battery performs the role of adjusting the entire power supply according to demand by holding surplus power generated by a photovoltaic power generator, a wind power generator, and a diesel power generator, or by discharging power when supply is insufficient even though the photovoltaic power generator, the wind power generator, and the diesel power generator are operated. In other words, the battery performs the role of maintaining the frequency of the stand-alone micro-grid.
However, there is a limit for the battery in terms of charge capacity (kWh) and discharge capacity (kWh). Accordingly, there is no major problem in adjusting power supply according to demand within the short term, but unless special measures are taken for long term, there may be problems with the overall adjustment of electricity supply according to demand.
For example, when there is sufficient wind and sunlight so that a power amount generated from renewable energy is greater than a load, a charge amount of a battery will continuously increase. As a result, the battery becomes fully charged, and the battery is no longer able to store remaining energy. In such a case, output power obtained from renewable energy source is forcibly reduced or stopped, or a dummy load is synchronized to consume the energy.
Alternatively, when there is insufficient or sunshine due to cloudy weather so that an output from renewable energy source is too low and the battery discharges power, a charge amount of the battery becomes low and a diesel power generator is operated to ensure sufficient power. In order to adjust power supply according to demand in the above cases, conventionally, an additional operational system that is identical to an energy market system (EMS) is implemented so that the system autonomous performs necessary actions.
However, when implementing an additional EMS, problems arise in terms of efficiency and economy. In other words, in order to implement an EMS and designing a stand-alone micro-grid in a small island area, implementation of communication infrastructure for all power generators and loads is required, and each of power generators and loads has to include a communication function. In addition, implementation of an expensive operational system is required, and a database of the corresponding site, development in human machine interface (HMI) etc., are also required, thereby taking a long time and inquiring high cost.
Accordingly, when designing a stand-alone micro-grid, a method of adjusting power supply according to demand of an independent system without implementing an EMS is required, and, rather than using a control method and a central data processing based on an EMS, a system and method of autonomously controlling a stand-alone micro-grid system wherein respective apparatuses provided for power supply according to demand, such as diesel power generators, power generators using renewable energy, loads, etc. are autonomously controlled by a frequency output from a battery system, is required.